Nitrogen oxides (NOx) comprise nitrogen monoxide (NO) and nitrogen dioxide (NO2). Reference herein to NOx is to a mixture of nitrogen oxides including NO and NO2, and references herein to NO and NO2 specifically should be interpreted accordingly.
In our International patent application no. PCT/GB2004/001680 (the entire contents of which are incorporated herein by reference) we describe a novel method of decomposing NO2 to NO in an exhaust gas of a lean-bum internal combustion engine, which method comprising adjusting the C1 hydrocarbon:nitrogen oxides (C1 HC:NOx) ratio of the exhaust gas to from 0.1 to 2.0 and contacting the gas mixture with an acidic metal oxide selected from the group consisting of zeolites, tungsten-doped titania, silica-titania, zirconia-titania, gamma-alumina, amorphous silica-alumina and mixtures of any two or more thereof, and passing the effluent gas to atmosphere.
One application of such a method is to decompose NO2 generated over an oxidation catalyst or a catalysed soot filter before the exhaust gas is passed to atmosphere. Such an application is often described as a “clean-up” catalyst. In one embodiment described in PCT/GB 2004/001680, the oxidation catalyst is a NO oxidation catalyst in Johnson Matthey's CRT® system described in EP 0341832 or U.S. Pat. No. 4,902,487 (both of which are incorporated herein by reference) for oxidising NO to NO2 for combusting particulate matter (PM) collected on a filter in NO2 and the clean-up catalyst is located downstream of the filter, or on the downstream end of the filter. The CRT® filter can also contain a precious metal catalyst to promote PM combustion.
In PCT/GB2004/001680 we explain our belief that the HC reductant is forming a coke on the acidic metal oxide and it is the coke that, at least in part, promotes the decomposition of NO2 to NO.
EP 0541271 (incorporated herein by reference) discloses a catalyst system for treating NOx in the exhaust from a lean-bum gasoline-fuelled engine, which system comprising a first stage catalyst containing a transition metal-exchanged zeolite (i.e. Cu-ZSM5), and a second stage catalyst, which is a three-way catalyst, for treating the effluent from the first stage catalyst. The engine is controlled such that the ratio of NOx to HC in the exhaust gas is in the range of from 1/3 to 3/1 (i.e. minimum C3H6 of 250 ppm (or 750 ppm C1) and a minimum NOx of 200-400 ppm). EP 0541271 does not disclose the method of PCT/GB2004/001680 because the effluent gas of the first stage catalyst undergoes further treatment in the second stage catalyst, whereas the effluent gas of the method of PCT/GB2004/001680 is passed to atmosphere. Moreover, preferably the method of PCT/GB2004/001680 is for treating exhaust gas from a diesel-fuelled engine.
In our WO 03/033118 (incorporated herein by reference) we disclose an exhaust system for an internal combustion engine comprising a first lean NOx catalyst (LNC) comprising a metal on a support comprising alumina, titania, zirconia, non-zeolite silica alumina or mixtures or mixed oxides of any two or more thereof and a second, platinum-based LNC disposed with and/or downstream of the first LNC, wherein the exhaust system comprises means for coking the first LNC during normal engine operation. The invention of PCT/GB2004/001680 is novel over this disclosure for similar reasons to EP 0541271.
U.S. Pat. No. 6,202,407 (incorporated herein by reference) describes a method of catalytically reducing NOx to N2, i.e. lean NOx catalysis, using pulsed injection of hydrocarbon reductant. Preferred catalysts are amphoteric and include gamma-alumina, Ga2O3 and ZrO2, all optionally metallised with Cu, Ni or Sn.
A problem with the use of oxidation catalysts and catalysed soot filters is that, as exhaust emission legislation tightens, legislative bodies have begun to discuss limiting the amount of NO2 it is permissible to exhaust to atmosphere. For example, the California Air Resources Board (CARB) has proposed that a maximum of 20% of tailpipe NOx of the relevant drive cycle is emitted as NO2 (See California's Diesel Risk Reduction Program, September 2000 and Title 13, California Code of Regulations, Chapter 14, section 2706.). NO2 is toxic and can cause headaches, dizziness and nausea in low doses. It also has an objectionable smell. In the context of the CRT®, if there is insufficient PM on the filter to react with NO2 generated over the oxidation catalyst or the temperature of the exhaust gas is below a preferred range for combustion of PM in NO2, NO2 can slip past the filter and be undesirably exhausted to atmosphere.
This problem is particularly acute when internal combustion engines are used in confined spaces, such as mines, where vehicles are used to drill for, load, and transport mined material to the surface. Many mining operations generate PM, and so exhaust aftertreatment systems comprising filters for reducing the levels of PM emitted are being considered. Furthermore, explosives used to blast rock to recover a desired ore can generate NO2. Accordingly, it would be an advantage to reduce the exhaust gas emissions of both PM and NO2 to the atmosphere in closed environments to improve the health and safety of miners. Indeed, the US Mine Safety and Health Administration prevents the use of diesel exhaust systems comprising diesel particulate filter systems that increase NO2 emissions.